Work under the auspices of this grant is focused upon the structural study of membrane bound proteins such as the acetycholine receptor. Canthanide ions (LN plus 3) are being used as substitutes to study the bonding Ca plus 2 ions to the receptor molecules. Anomalous scattering of the LN 3 ions is maximized by the use of synchrotron radiation and tuning of the incident wavelength to the absorption resonance at the L3 edge. Such studies are expected to lead to a molecular picture of the binding sites for Tb plus 3/Ca plus 2 ions on the receptor. Anomalous scattering techniques are also being developed to determine distances between pairs of metal atoms on macromolecules in solution.